1. Field of the Invention
The present invention relates to an apparatus for compensating the luminance level of a video signal.
2. Description of the Related Background Art
Recently, since display units tend to be upsized, a luminance level compensating apparatus is required for displaying images more clearly. The luminance level compensating apparatus includes non-linear amplifying means for compensating the luminance level of a video signal. By supplying the video signal to the non-linear amplifying means, it is possible to extend the luminance distribution of the video signal to a dynamic range.
FIG. 1 shows a conventional luminance level compensating apparatus. The luminance level compensating apparatus includes a histogram memory 1, a maximum compensation value calculation circuit 2, and a look-up table memory 3. A digitized luminance signal as the input signal is supplied to the histogram memory 1 and the look-up table memory 3. The histogram memory 1 is a memory for storing the frequency data at each luminance level of the input luminance signal. The histogram memory has a plurality of storage areas each of which is addressed in accordance with each predetermined luminance level. The frequency data is stored in each of the storage areas. That is, at each time the luminance signal for one pixel is supplied to the histogram memory, frequency data in the storage area corresponding to that luminance level in the histogram memory increases by 1. All of the contents stored in the histogram memory 1 are cleared to zero (0) at every predetermined period (one vertical scanning period or its multiple). After clearing the histogram memory 1, new frequency data is then recorded for each luminance level.
The maximum compensation value calculation circuit 2 includes a histogram accumulation circuit 2a for accumulating data in the histogram memory 1 sequentially starting from the lowest luminance level, an accumulation histogram memory 2b for storing the result of the accumulation circuit 2a, and a normalization arithmetic circuit 2c for normalizing each data so that its maximum accumulation frequency becomes the maximum value of an output luminance signal based on data stored in the accumulation histogram memory 2b. Like the histogram memory 1, the accumulation histogram memory 2b has storage areas for storing frequencies which are addressed at the luminance level of each of a plurality of luminance levels of the luminance signal.
The look-up table memory 3 stores data obtained by normalizing data stored in the accumulation histogram memory 2b. Each address of the look-up table memory 3 is designated in accordance with a luminance level of the input luminance signal. A luminance level stored in the storage area corresponding to the designated address is output as a normalized level.
FIGS. 2A-2C show waveforms in luminance conversion by the conventional luminance level compensating apparatus. One of the addresses of the histogram memory 1 is designated for each pixel of the input luminance signal, and the frequency data value of the storage area corresponding to the designated address increases by 1. Assume that the frequency for the luminance levels of the input luminance signal in a predetermined period is detected as shown in FIG. 2A. Also, assume that the frequency at the luminance levels of Y150, Y160, Y170, Y180, Y190, Y200 and Y210 is detected in the histogram memory 1. In this case, the frequency has the relationship of Y150<Y160<Y170<Y180<Y190<Y200<Y210. Further, assume that the frequencies at the luminance levels of Y150, Y160, Y170, Y180, Y190, Y200 and Y210 in the predetermined period are 1, 3, 5, 7, 5, 3, 1; then the accumulated frequencies at the luminance levels of Y150, Y160, Y170, Y180, Y190, Y200 and Y210 are 1, 4, 9, 16, 21, 24 and 25. That is, as shown in FIG. 2B, as the luminance level increases, the accumulated frequency also increases. A normalization coefficient is calculated in the normalization arithmetic circuit 2c such that the maximum value of the accumulated frequency becomes the maximum value of an output luminance level. A normalization operation is performed on the basis of the normalization coefficient and each data value stored in the histogram memory 1. Each values obtained by the operation is stored in the corresponding storage area in the look-up table memory 3. FIG. 2C shows the relationship between the input luminance level and the output luminance level of the look-up table memory 3. By compensating the input luminance signal with reference to the look-up table memory 3, a luminance signal of which gradation is compensated is output.
However, in the conventional luminance level compensating apparatus, as an image produced by an input video signal, there is a narrow image such as the cinemascope size which is narrow in the vertical direction in an actual image. The image has blank areas (black areas) as shown by hatching in FIG. 3 at its top and bottom. Therefore, if the detection area of the accumulated histogram is an area enclosed by a dotted line A including the blank areas in FIG. 3, the accumulated histogram has the characteristic shown in FIG. 4. That is, the accumulated histogram has the frequency of the luminance level of an actual image greatly affected by the frequency of the black level of the blank areas. If the luminance level is compensated in gradation on the basis of the accumulated histogram, a problem such that black remarkably appear on the actual image occurs.